Water-based paint formulations containing mineral fillers consist of an aqueous phase, one or more polymers in emulsion in the liquid phase called binders, fillers and/or pigments, a dispersing agent and admixtures as diverse as surfactants, coalescence agents, biocides, anti-foaming agents and, finally, at least one thickening agent.
The latter enables the rheology of the water-soluble formulations into which it is introduced, and notably water-soluble paints, to be controlled, both at the manufacturing stage, and during transport or storage, or at the time of application. The wide variety of practical constraints in each of these stages relates to a multiplicity of different rheological properties.
Nevertheless, it is possible to summarise the requirement of the skilled man in the art in obtaining an effect of the thickening of the water-based formulation, both for reasons of stability over time, and for a possible application of the paint to a vertical surface, lack of spattering during use, etc. For this reason additives have been designated which contribute to this regulation of the rheological properties under the term “thickening agents”.
Among these products, thickening agents known as “associative” thickening agents are distinguished, which are hydrosoluble polymers having insoluble hydrophobic groups. Such macromolecules have an associating character: when introduced into water, the hydrophobic groups tend to assemble in the form of micellar aggregates. These aggregates are linked together by the hydrophilic parts of the polymers: a three-dimensional network is then formed which causes the viscosity of the medium to be increased.
The functional mechanism and the characteristics of the associative thickening agents are now well known and described, for example in the documents “Rheology modifiers for water-borne paints” (Surface Coatings Australia, 1985, pp. 6-10) and “Rheological modifiers for water-based paints: the most flexible tools for your formulations” (Eurocoat 97, UATCM, vol. 1, pp 423-442).
Among these associative thickening agents the class of associative thickening agents of the HEUR type (Hydrophobically modified Ethylene oxide URethane) is distinguished. They designate copolymers resulting from the synthesis between a compound of the polyalkylene glycol type, a polyisocyanate, and a monomer or condensate designated “associative”, of the alkyl, aryl or aryalkyl type consisting of a hydrophobic terminal group.
These structures are well known to develop high viscosities, with a medium to low shearing gradient (J. of Applied Polymer Science, vol. 58, p 209-230, 1995; Polymeric Mat. Sci. and Engineering, vol. 59, p 1033, 1988; Polymeric Mat. Sci. and Engineering, vol. 61, p 533, 1989; Polymeric Paint Colour Journal, vol. 176, no. 4169, p 459, June 1986), which is equal respectively to the Stormer™ (KU) and Brookfield™ (mPa·s) viscosity measurements.
At the same time, document EP 1 566 393 describes a thickening agent of the HEUR type one of the essential characteristics of which is the presence of n-butyl-1-octanol, while its hydrophobic groups are based on fatty alcohols having 8 to 18 carbon atoms.
Document EP 1 013 264 describes a polyurethane thickening agent for cosmetic formulations, having an associative monomer functionalised by a hydrophobic group which may be linear or branched, but is preferentially linear, and having 12 to 24 carbon atoms.
Document WO 94/06840 proposes an associative thickening agent of the HEUR type, characterised by a certain density of hydrophobic groups, where the said groups are linear alkyl chains having 8 to 22 carbon atoms.
Document EP 1 584 331 proposes a hydrophobic terminal group having 6 to 34 carbon atoms for the associative monomer. To increase specifically the Brookfield™ viscosity.
Document EP 0 639 595 proposes linear hydrophobic groups having 4 to 36 carbon atoms.
Document WO 02/102868 also makes reference to linear structures for the associative monomer.
Independently of the particular rheological profiles provided by the variations described above, the HEURs with the most pronounced thickening power still remain molecules having alkyl phenols grafted on to their associative monomer. One of the representative products of this technology is Acrysol™ SCT-275, developed by the company DOW™.
And alkyl phenols are currently widely suspected of being carcinogenic, and dangerous for reproduction; although still tolerated in the paints industry they nonetheless remain in the firing line of the legislative institutions, notably the European ones.
There is therefore a genuine requirement to develop a HEUR-type associative thickening agent, which is free of alkyl phenols, but which has an equivalent or even improved thickening power. The inventors have discovered, in a completely surprising manner, that the use of certain structures in place of the associative monomer led to such a result.